There is today considerable need for an economical and efficient method of removing ions of metals from solutions in order to effect recovery of the metals. Such a need is presented, for instance, in the extraction of metal ions from ore leach liquors produced by in situ mining or dump leach extraction of copper and uranium. Other examples where such a need is evidenced is in the treatment of industrial water effluent streams, and in the treatment of electroplating rinse baths, where it is desired to recover such ions as iron, nickel, copper, chromium, etc. In some instances, it is desirable to extract a single type of ion, such as the ions of a single metal, and in others an entire class of metals.
Solvent extraction with organic complexing agents has been used in ion recovery processes. However, the cost of such a process is usually high, thereby limiting application of the process to only a few favorable situations.
Membranes have also been used in the past in various types of recovery systems. For instance, membranes have been employed in a so-called solution-diffusion process. In this process, transport through the membrane occurs by reason of dissolution of the permeating species in the membrane at one interface and diffusion down a gradient in thermodynamic activity. A problem inhibiting wide acceptance has been the lack of sufficient selectivity, and a low transport rate across the membrane. A process known as "facilitated transport", utilizing a membrane to effect separation, is also known. A problem with this type of process is that it is limited to neutral species. As a consequence, so-called facilitated transport in general has been applied only to the separation of gases.
Coupled transport refers to another process, similar to but less understood than facilitated transport, which may be employed to effect ion removal from an aqueous solution with the ion transported across a membrane. Coupled transport, as described in greater detail below, employs a membrane impregnated with a liquid which includes a complexing agent. In a coupled transport process electrical neutrality is preserved by the movement of other ions through the membrane in addition to the ions that are being recovered.